The present invention relates to a composite body of silicon carbide and binderless allotropic carbon, and to the process of making such a composite body. More specifically, it relates to making such composite bodies for use in tribological applications such as mechanical seals, bearings and other sliding or rubbing components, which require good durability and wear performance under less than ideal lubricating conditions.
Material science has long been used as a basis for choosing materials for components having surfaces which are in or may come into sliding contact with each other. For example, in the field of mechanical face seals, a primary ring made of a carbon or carbon like material and a mating ring made of another material such as silicon carbide, tungsten carbide, alumina, stainless steel, etc., are generally known. However, in mechanical face seals exposed to highly abrasive fluids, the primary ring needs to be made of more abrasion resistant material than carbon. In such situations, a special silicon carbide may be run against a hard material such as another silicon carbide. In order to improve tribological compatibility of these hard face materials, especially at higher pressures, special silicon carbide grades have been developed. Since silicon carbide has relatively poor lubricity, the use of graphite incorporated into the matrix has been pursued by many inventors.
U.S. Pat. No. 4,536,449 by Kennedy et al, issued on Aug. 20, 1985, describes the addition of graphite to silicon carbide utilizing the reaction bonding process. Unfortunately, it is difficult to control the graphite size and content due to the normal presence of 2 to 20 percent highly reactive, free silicon.
Boecker et al describes a graphite containing silicon carbide produced by pressureless sintering in U.S. Pat. No. 4,525,461, issued on Jun. 25, 1985. This approach utilizes addition of small graphite particles not exceeding an average size of 8 micrometers. However, the addition of the amount and size of the graphite particles is limited by this procedure due to the inherently high shrinkage associated with the direct sintering process.
Okuno et al describes a silicon carbide-graphite composite material in U.S. Pat. No. 4,701,426, issued on Oct. 20, 1987, and in U.S. Pat. No. 4,690,909, issued on Sep. 1, 1987. According to these patents, graphite or carbon black added to the silicon carbide yielded a composite material where graphite having an average grain size of not more than 3 micrometers is uniformly dispersed as a secondary phase along the grain boundaries of all the silicon carbide grains.
Moehle et al describes a graphite containing silicon carbide in U.S. Pat. No. 4,942,145, issued on Jul. 17, 1990. This approach is based upon using an organosilicon binder such as polysilazanes, silicon carbide, and graphite fillers. The graphite containing silicon carbide is mixed with polysilazane dissolved in an organic solvent, dried, molded and pyrolyzed around 1300xc2x0 C. In one example, such graphite containing silicon carbide sample was found to have 2.18 gM/CM3 density and a bending strength of 15.9]xcx9cg/MM2 (22,600 psi).
Talbert et al describes a graphite containing silicon carbide in U.S. Pat. No. 5,543,368, issued on Aug. 6, 1996, and U.S. Pat. No. 5,486,496, issued on Jan. 23, 1996. According to these patents, first the graphite powder is mixed with a binder such as polyethylene glycol and other additives to facilitate uniform dispersion of the graphite in the slurry. The size of the graphite was disclosed in the range of 3 to 4 micrometers. The graphite containing slurry is then spray dried to the desired size distribution of the graphite agglomerates. According to these patents, the preferred average spherical graphite agglomerate size is reported to be around 100 micrometers. Separately, the silicon carbide molding powder using sub-micrometer silicon carbide and the appropriate additives is produced by similar spray drying technique. Next, the desired amount of graphite agglomerates are added and blended with the spray dried silicon carbide molding powders. Finally, the desired graphite containing silicon carbide composite parts are processed according to teaching of U.S. Pat. No. 4,041,117 by Prochazka, issued on Aug. 9, 1977. As a result of using loosely bonded graphite agglomerates instead of solid graphite particles, the problem of adding graphite to the inherently high shrinkage self-sintered silicon carbide has been solved. However, the reproducibility of the material""s tribological performance may be adversely affected by such an approach.
Chen et al describes a graphite containing silicon carbide in U.S. Pat. No. 5,422,322, issued on Jun. 6, 1995, and U.S. Pat. No. 5,656,563, issued on Aug. 12, 1997. The process for producing a graphite containing silicon carbide is described in U.S. Pat. No. 5,976,429, issued on Nov. 2, 1999. According to these patents, first the 45 micrometer size graphite is mixed with a phenolic resin dissolved in an organic solvent such as acetone. Then the mixture is dried to evaporate the solvent and crushed to pass through a 200 mesh screen. Next the desired amount of resin bonded graphite is added to the slurry comprised of sub-micro meter silicon carbide and associated additives. The slurry is spray dried to form spherical granules having an average size of less than 500 micrometers. The spray dried molding compound is pressed to the desired shape, carbonized to 850T, and sintered at preferably 2000 to 2200xc2x0 C. in inert atmosphere such as helium or argon. The sintered silicon carbide-graphite composite contains between 2 to 30 percent by weight carbon bonded graphite. The composite, especially at higher carbon bonded graphite concentrations, contains structural defects described in the patent as microcracks. The shape of the carbon bonded graphite particles is also irregular, which maximizes stress concentrations in the sintered composite material. Furthermore, the large and uncontrolled particle size distribution of the resin coated graphite affects the reproducibility of the inherently high shrinkage of the silicon carbide matrix.
Graphite containing silicon carbides produced by different methods discussed in the prior art appear to result in a robust material with the process characteristics for highly demanding tribological applications.
The invention disclosed and claimed herein comprises a composite body of silicon carbide having binderless allotropic carbon distributed throughout, a process for producing such a composite body, and an article of manufacture for use in tribological applications, which utilizes such a composite body. The steps for producing the composite body include forming a mixture of silicon carbide and binderless, allotropic carbon-yielding precursor granules, then shaping and heating the mixture to form the desired composite body. Articles made according to the present invention can be produced in a variety of shapes for use in tribological applications such as mechanical face seals.